1. Field of the Invention
The present invention relates to a plasma reaction apparatus, and, more particularly, to a plasma reaction apparatus for etching the surface of a semiconductor substrate with a gas discharge.
2. Description of the Related Art
When a semiconductor device is manufactured, the semiconductor substrate is subjected to processes such as a thin film forming process and an etching process. A semiconductor substrate processing apparatus, may generate a plasma in a gas discharge.
FIG. 5 is a schematic cross sectional view which illustrates an ordinary conventional plasma reaction apparatus 100 in which a plasma is generated by ECR (Electron Cyclotron Resource) discharge.
Referring to FIG. 5, the plasma reaction apparatus 100 comprises a reaction chamber 1, and a waveguide tube 6 for introducing microwaves into the reaction chamber 1, a quartz plate 7 serving as an introduction port through which the microwaves are introduced. A single solenoid coil 4 serving as a magnetic field generating means and a mirror coil 5 are disposed outside the reaction chamber 1, the single solenoid coil 4 surrounding the reaction chamber 1. A gas introduction port 8 is formed in the upper portion of the reaction chamber 1 and an exhaust port 13 is formed in the bottom portion of the reaction chamber 1. Furthermore, the reaction chamber 1 includes a supporting frame 3 on which a semiconductor substrate 2 is placed.
The thus structured apparatus is operated as follows. The inside portion of the reaction chamber 1 is exhausted by means of a vacuum pump (omitted from illustration) through the exhaust port 13. Then, a reactive gas is introduced into the reaction chamber 1 through the gas introduction port 8, and a portion of the reactive gas is exhausted through the exhaust port 13 while continuing the gas introduction into the reaction chamber 1 so as to make the gas pressure constant. Furthermore, microwaves having a frequency of 2.45 GHz and generated by a microwave power source (omitted from illustration) are introduced into the reaction chamber 1 via the waveguide tube 6 and the quarts plate 7. On the other hand, electricity is supplied to the single solenoid coil 4 disposed around the reaction chamber 1 so as to cause the solenoid coil 4 to generate a magnetic field for exciting ECR, that is a magnetic field having a magnetic flux density of 875 gauss in the reaction chamber 1. Also the mirror coil 5 is supplied with electricity so that it generates a magnetic field in the same direction as that generated by the solenoid coil 4. As a result, a weak mirror magnetic field is created between the solenoid coil 4 and the mirror coil 5 so that magnetic lines of force are perpendicular to the surface of the semiconductor substrate 2.
As a result of the above-mentioned operation, reactive gas molecules in the reaction chamber 1 are formed into a plasma due to collisions with electrons accelerated by the ECR. The reactive gas plasma thus generated is dispersed along the magnetic lines of force so that the dispersed reactive gas plasma is perpendicularly incident on the surface of the semiconductor substrate 2 on the supporting frame 3. At this time, the surface of the semiconductor substrate 2 is etched in a desired direction. The type and the pressure of the gas, and the microwave power and the like are determined according to the process to which the semiconductor substrate 2 to be processed is subjected.
In the plasma reaction apparatus 100 utilizing the ECR discharge, the resonance region is not widely and uniformly formed because the magnetic field required for the electron cyclotron resonance is generated by the single solenoid coil 4. That is, it has been difficult to obtain an etching apparatus revealing a high etching rate and satisfactory uniformity. The reason for this will now be described with reference to FIG. 6 which is a graph showing the relationship between solenoid electric current values and etch rates while also referring to uniformity data.
When the electric current supplied to the solenoid coil 4 is small, that is, about 160A as shown in FIG. 7, the gradient of the magnetic field is small in the ECR region in the vicinity of the center of the reaction chamber 1 and the gradient of the magnetic field increases in proportion to the distance from the center. Therefore, a non-uniform resonance region distribution is (as will be described later with reference to FIG. 7) displayed as follows: the resonance region is distributed widely in the center portion of the reaction chamber 1 and narrowed in inverse proportion to the distance from the center as shown by the diagionally-lined area 11 in FIG. 5. On the other hand, in a case where the value of the electric current supplied to the solenoid coil 4 is large, that is, about 190A,. a uniform ECR region is formed as shown by the diagonally-lined area 12 in FIG. 5 because the gradient of the magnetic field in the ECR region is great (as will be described later with reference to FIG. 7). However, the uniform magnetic field has been too narrow and therefore only a narrow resonant region has been formed.
Results of experiments carried out by using the conventional plasma reaction apparatus will now be described with reference to FIGS. 6 and 7.
The specifications of the coil used in the experiment are as follows: the inner diameter was 27 cm, the outer diameter was 39 cm, the axial directional length was 14 cm and the number of turns was 160. An Si-substrate having a diameter of 6 inches was etched by a plasma generated under conditions that the flow rate of chlorine gas was 10 cc/minute, the pressure was 0.5 mTorr and the microwave power was 600 W.
According to the magnetic field characteristics shown in FIG. 7, the gradient of the magnetic field parallel to the axis of the magnetic field is small when an electric current having a small value is supplied to the coil. The value of the axial gradient along the central axis (R=0 cm) and at a point (R=10 cm) 10 cm distant from the central axis are considerably different from each other.
Since the magnetic field gradient and the area of the ECR region are in inverse proportional relationship, a wide ECR region is formed when the magnetic field gradient is small. As a result, the density of the generated plasma is increased. Therefore, the etch rate is raised in a case where the value of the electric current supplied to the coil is small as shown in FIG. 6.
Although it is preferable that the practical etch rate be 1000.ANG./min or higher, the uniformity of the magnetic field becomes worse, excessively so, to 20% or more, and the ECR region is distributed as shown by the diagonally-lined area 11 in FIG. 5 when the etch rate is increased. As a result, desired uniformity of etching on the semiconductor substrate 2 cannot be obtained.
On the other hand, if a uniform ECR region (represented by reference numeral 12 shown in FIG. 5) is desired (adjacent to the coil electric current of 190A shown in FIG. 6), the ECR region is narrowed and a practical etch rate of 1000.ANG./min cannot be obtained.
As described above, the conventional plasma reaction apparatus using ECR therefore encounters a problem that an increase in the etching speed and an improvement of the uniformity of the magnetic field cannot be realized simultaneously.
That is, as can be understood from the above description, the axial gradient of the magnetic field and the etch rate have a close relationship. Furthermore, the uniformity of the gradient of the magnetic field must be improved in order to improve the uniformity of the etch rate. The radial directional uniformity of the gradient of the magnetic field shown in FIG. 7 is calculated by the following equation 1 from the larger and the smaller of the gradients of the magnetic field in a region at a central axis (R=0) to the point of 10 cm distant from the central axis (R=10 cm): ##EQU1##
As can be seen from a dashed curve shown in FIG. 7, increasing the electric current supplied to the coil improves the uniformity of the gradient of the magnetic field and coincides with an increase in the uniformity of the etch rate as shown in FIG. 6.
However, there arises a problem in that, if an enlarged electric current is supplied to the coil in order to improve the uniformity of the gradient of the axial magnetic field, the value of the gradient of the magnetic field is undesirably increased and therefore the etch rate deteriorates.